Temporal Changes in Soil Carbon Stability at a Paired Old-Growth Douglas-Fir Forest/Clear-Cut Site.

Forest ecosystems contain approximately one-half of the Earths terrestrial carbon (C) (1146 Pg), with two-thirds (787 Pg) of this pool residing in forest soils. Given the magnitude of the soil C pool, it is critical to understand the effects of forest management practices on soil C dynamics. The main objective of this study was to assess the temporal effects of clear-cutting on micro-climatic and soil-climatic parameters and its subsequent impact on soil C stability. The study was conducted at a paired old-growth Douglas-fir forest and clear-cut site located in the Cascade Range of western Oregon. The clear-cut site was cut in 1991 and replanted in 1994. Temporal changes in soil temperature, soil moisture, and climatic variables (e.g., air temperature, precipitation) were measured continuously at hourly intervals over a 13-year period (1996-2009). Soil samples were collected from the clear-cut site at 2, 3, 5, 7, and 19 years post-clear-cut and soil samples from the adjacent reference forested site were taken at 0, 9, and 19 years post-clear-cut. Temporal changes in soil C stability were assessed using thermogravimetry-differential scanning calorimetry (TG-DSC). TG-DSC assesses soil weight loss and energy evolution continuously across a 0-1000°C temperature range. Results from soil climatic data indicate higher summer soil temperatures at 5 and 30 cm depths at the clear-cut relative to the mature forest site. Soil moisture was similar between sites at the 0-20 cm depth, while at the 20-40 cm depth peak soil moisture (Nov-May) was generally higher at the clear-cut site. Preliminary results from thermal analysis indicate a reduction of soil C mass and energetic storage in the labile (i.e., 200-350 °C) pool for the harvested site compared to the reference. Together these results suggest that clear-cutting alters site temperature and moisture post-harvest and may alter aspects of SOM stability and storage over decadal time-scales.